The present invention is directed to a radio receiver including a device for down-converting a received radio signal modulated by information signals to an intermediate frequency signal by using a local oscillator, and a demodulating device for recovering the information signals from the intermediate frequency signal. The radio receiver of the present invention is adapted to receive radio signals in at least two frequency bands.
The present invention also is directed to a corresponding radio transmitter adapted to transmit radio signals in at least two frequency bands, the radio transmitter including a device for modulating an intermediate frequency signal by information signals, a device for converting the intermediate frequency signal to a radio signal by using a local oscillator, and a device for transmitting the radio signal.
In known receivers and transmitters, of which the superheterodyne receiver and corresponding transmitters are examples, the frequency of the local oscillator can be selected either above or below the frequencies of the relevant frequency bands. For each band the frequency of the oscillator should be variable over a range corresponding to the range of the band, because the difference in frequency between the radio signals and the local oscillator should be equal to the frequency of the intermediate signal, which has a very low bandwidth around a fixed frequency. However, in many situations the frequency bands cover a wide frequency range. This is the case, for example, with respect to multiband mobile telephones. Such telephones could for instance be adapted to receive and/or transmit signals in the GSM900 band (900 MHZ) and the GSM1800 band (1800 MHZ). If the same local oscillator should be used for the two bands in this case, the oscillator should be variable over a range of at least 900 MHZ, which would be very difficult to implement in practice. Therefore, it will normally be necessary to use a separate local oscillator for each band.
However, the use of two or more separate local oscillators complicates the design of the receiver or the transmitter considerably, and, especially, it takes up more space, which is a critical point in mobile telephones, because much effort is devoted to reducing the size of the telephones.
An object of the present invention is, therefore, to provide a receiver and a transmitter which are able to receive/transmit signals in at least two frequency bands with the use of only one single local oscillator.
In accordance with the present invention, this object is achieved in that the frequency of the local oscillator is selected so that the frequencies of at least one of the frequency bands are above the frequency of the local oscillator, and the frequencies of at least one of the frequency bands are below the frequency of the local oscillator.
When the frequency of the local oscillator is selected to be between the relevant frequency bands, i.e. above the frequencies of the lower band (e.g., 900 MHZ) and below the frequencies of the upper band (e.g., 1800 MHZ), the range over which a single oscillator should be variable (i.e. the bandwidth of the oscillator) is reduced considerably. Therefore, it is possible to implement such an oscillator in practice.
When the receiver or the transmitter of the present invention is adapted to receive or transmit radio signals in only two frequency bands and the frequency of the local oscillator is selected in a range substantially halfway between the two frequency bands, the range over which the single oscillator should be variable is reduced even further to a range of substantially the same size as for each band, because approximately the same oscillator frequencies may be used for both bands. If, for instance, the two bands are the earlier-mentioned GSM900 and GSM1800 bands, the intermediate frequency could be selected to be approximately 450 MHZ, causing the frequency of the local oscillator to be in a range of around 1350 MHZ for both bands.
With respect to the receiver, it is well known that the selection of a local oscillator frequency above the frequencies of the relevant frequency band for the received radio signals causes sideband reversal in the down-converted signal, so that the upper side band at radio frequency becomes the lower side band at the intermediate frequency, and vice versa. This side band reversal does not occur when the frequency of the local oscillator is below the frequencies of the radio frequency signals. When the local oscillator frequency is selected between the two bands, a switch from reception in one band to reception in the other band will therefore also cause a change of the side bands in the intermediate frequency signal. This change of sidebands must be compensated in the receiver, for instance in the demodulating device.
When the demodulating device is a quadrature demodulator adapted to recover the information signals in the form of an In-phase signal (I signal) and a Quadrature-phase signal (Q signal), and when the modulation of the received radio signals is a phase modulation or a frequency modulation, this compensation may be implemented by implementing a receiver that includes a device for assigning one sign (+or xe2x88x92) to the Q signal when the frequency of the received radio signal is below the frequency of the local oscillator, and the opposite sign when the frequency of the received radio signal is above the frequency of the local oscillator. It can be shown that in the quadrature demodulator the change of side bands means a change of the sign of the Q signal and, therefore, the change may be compensated by changing the sign back.
Further, it can be shown that a change of sign of the Q signal is equivalent to swapping or exchanging the I signal and the Q signal. Therefore, the compensation of the change of sidebands may be implemented in that the receiver moreover comprises a device for keeping the I signal and the Q signal unchanged when the frequency of the received radio signal is above the frequency of the local oscillator, and for exchanging the I signal and the Q signal when the frequency of the received radio signal is below the frequency of the local oscillator.
In the transmitter case a similar effect is present. The selection of a local oscillator frequency above the frequencies of the relevant frequency band for the transmitted radio signals causes sideband reversal, so that the upper side band at intermediate frequency becomes the lower side band at the radio frequency, and vice versa. Also here, the side band reversal does not occur when the frequency of the local oscillator is below the frequencies of the radio frequency signals. When the local oscillator frequency is selected between the two bands, a switch from transmission in one band to transmission in the other band will therefore also cause a change of the side bands in the radio frequency signal. This change of sidebands must be compensated in the transmitter.
When the modulating device is a quadrature modulator adapted to modulate the intermediate frequency signal by information signals in the form of an I signal and a Q signal, and when the modulation of the received radio signals is a phase modulation or a frequency modulation, this compensation may be implemented by a transmitter that includes a device for assigning one sign (+or xe2x88x92) to the Q signal when the frequency of the radio signal is below the frequency of the local oscillator, and the opposite sign when the frequency of the radio signal is above the frequency of the local oscillator. As with the receiver, it can be shown that in the quadrature modulator a change of the sign of the Q signal corresponds to a change of side bands in the intermediate frequency signal and, therefore, such a change can compensate the sideband change that occurs when the intermediate frequency signal is converted to the radio signal.
It can be shown that a change of sign of the Q signal is equivalent to swapping or exchanging the I signal and the Q signal. Therefore, the compensation of the change of sidebands may be implemented in that the transmitter moreover comprises a device for keeping the I signal and the Q signal unchanged when the frequency of the radio signal is above the frequency of the local oscillator, and for exchanging the I signal and the Q signal when the frequency of the radio signal is below the frequency of the local oscillator.